There are already a large number of prior-art electrodes and similar devices for selectively determining ions in aqueous solutions. The electrodes usually comprise an internal electrochemical reference half-cell at a stable potential and adapted, in conjunction with an external reference electrode immersed simultaneously with the measuring electrode in a solution to be analyzed, to form a cell having a potential dependent on the presence of a given ion in the solution In some texts, an assembly of this kind comprising two electrodes and a connecting electrolyte is described as a "battery". We shall avoid using this term here, since a "battery" produces current in contrast to the present cell, which delivers a potential only.
The potential is proportional to the logarithm of the activity of the chosen ion in accordance with NERNST's well-known equation, one form of which, for example, is E=E.sub.o .+-.KlnC.sub.i where E.sub.o is the standard potential and K is a constant. The potential is therefore proportional to the concentration of the ion, which can then be deduced by comparison with standard solutions.
For example, documents CH-A-604 167 and US-A-4 214 968; 4 053 381; 4 171 246 (EASTMAN KODAK) describe electrodes in the form of dry multi-laminates comprising, in the following order, an insulating support, a metal electrically conductive layer covered with an insoluble salt of the metal, a part of the layer being used as a terminal for connection to an electrometer, a layer of reference electrolyte in a hydrophilic binder and an ion-selective diaphragm for selectively measuring a given ion, to the exclusion of other ions in the analyzed substance. The assembly made up of the metal layer, the insoluble salt and the reference electrode constitutes the internal reference half-cell at stable potential. Alternatively the assembly can be replaced by a conductive layer covered with a redox system, e.g. the quinone-hydroquinone couple, which system likewise comprises an internal reference cell at stable potential.
The ionically selective diaphragm usually comprises a plasticizer and a hydrophobic matrix containing in dispersed form an ionophoric substance, i.e. one of use for selectively detecting a given ion to the exclusion of any other ions in the analyzed solution. The reference electrolyte layer comprises a suitable water-soluble salt dispersed in a hydrophilic binder matrix containing 5 to 25% water. Compare W. E. MORF et al., Ion-Selective Electrodes in Analytical Chemistry, Vol. I. FRIESER Editor, Plenum Press (1981). pp 221 ff.
Document US-A-3 856 649 (MILES) describes an electrode having a similar structure except that the conductive element is filiform instead of laminar.
Document GB-A-2 106 253 (FUJI) likewise describes an electrode for selectively determining ions, the electrode being a laminate comprising an insulating support, a conductive layer covered with an insoluble salt of the metal forming the conductive layer, and a hydrophobic layer of an ion selective material (ISM) covering the rest. This document also describes a simplified variant electrode omitting not only the reference electrode but also the insoluble salt. In this variant the electrode comprises only the conductive layer covered with ISM material. With regard to this kind of electrode, see also Anal. Chem. 44, 856 (1972).
Document US-A-4 454 007 (DUPONT) likewise describes a laminar ion selective electrode having the following structure: a baseplate of insulating material, a layer of conductive material, a layer comprising powdered carbon dispersed in a dielectric polymer, and finally an ion-selective diaphragm made of material which penetrates with the material in the preceding layer at their junction plane.
In conventional practice, the previously-described electrodes are used as follows: When testing a solution for analysis, a drop thereof is deposited on to the diaphragm, which has selective ion permeability, and the drop is also placed in contact with the external reference electrode, e.g. via a salt bridge, the terminals of the ion-selective electrode and the reference electrode being connected to a suitable electrometer for reading the potential. In numerous prior-art cases, the external reference electrode may be similar to or identical with the measuring electrode, and the external reference potential can be supplied either by a standard solution deposited simultaneously with the solution to be analyzed (in which case the solutions make contact by diffusion into a porous element situated between the deposition areas) or by a fixed reference element (the salt bridge for example) forming part of the external reference electrode.
In order to locate and fix the position of the drop of solution to be analyzed (and also the drop of standard solution when necessary) the ISM diaphragm is usually covered with a layer of insulating, waterproof material formed with compartments or windows giving access to only that portion of the ISM layer facing the window. This system prevents the drop spreading on the surface of the ISM layer and can also be used for selecting a fixed preset quantity of liquid for measuring, since the capacity of the compartments is kept constant from one electrode to the other.
The internal reference cell in laminar electrodes is a metal layer covered successively by an insoluble salt of the metal and a reference electrolyte having the same anion as the insoluble salt and the same cation as the ions to be selectively measured. These electrodes are valued for their stability and reliability, but it is often preferable, for simplicity of manufacture, to use electrodes where the reference cell is a redox system. In such cases, the reference element is simply a conductive layer (a metal or other conductive material such as graphite or carbon) covered by the redox system, usually embedded in a hydrophilic matrix. A wide variety of systems are used as the redox couple, e.g. Fe.sup.+2 /Fe.sup.+3, quinhydrone, Fe(CN).sub.6.sup.-3 /Fe(CN).sub.6.sup.-4, etc. However, if the reference element is to operate efficiently, it is usually necessary for the redox medium to be moistened, either before using the electrode (moist electrodes) or at the time of use (storage of electrodes when dry). Usually also, if the element is to have the optimum redox capacity, the molar ratio of the substance in oxidized form to the substance in reduced form must be near unity, which necessitates certain precautions during manufacture.